Ive been searching around for a while about all the things i could possibly do to fix my fluctuating idle problem (between 1400 and 1700, which just started out of nowhere) and came across someone stating that there was something specific to check for if when you turned on the A/C It would stop it from fluctuating but he couldnt remember what it was. Can someone point me to this thread or tell me what it is??

o ya, Its a 91 N/A.

 

The first thing you need to do is get the idle down to within normal range. Once you get the idle down (this will tell you a lot just working through that) you'll have a lot of options to go through. I'd start in this order:

Check cable/linkages/cold start (is it hanging up/open?)
Check for vacuum leaks (I know, but it's HUGELY important)
Check the idle screw on the TB (close it all the way, back it out about 1 turn or so and see where it's at)
Check BAC, make sure it's not stuck open (if you even have it)
Once you get the idle somewhat within normal range (even if it's still hunting around) then check your TPS/timing/etc.

 

Ive been checking for vacuum leaks, and have yet to find any, but im still searchin, and i checked my tps with the two light method, and only the one was on, but im goin to test it with a meter and see what i get.

Also, i noticed that when i turn that screw on the TB all the way in, it stop jumping around and sits at a steady 1000 rpm.??

I still have my BAC on so im gonna take that off and clean it and see if that will help.

Also the car has about 70,000 on it and im not sure if the timing has been adjusted since the 60,000 mark. i got it with 67,000 on it.. Im just not sure how to even adjust the timing on these things yet lol

 

You need the idle below 1k before you can really do much. If you are ABSOLUTELY certain that you have no vac leaks/have checked everything else, you can adjust the throttle plates themselves to get your idle down. Typically I aim for about ~600rpm with the throttle plates and the screw closed, then i adjust it up with the bypass air screw ontop the TB to ~800, then i work out the issues with the TPS/etc. (I do all of this with the BAC not in operation)

 

ok, i have same problem.
gonna do the checks as well tomorrow.
but i can't remember anything, whats BAC?

 

bypass air control

 

oh, thanks.

 

What rock did you crawl out from under?

 

I don't recall, but it smelled terribly.

 

Well, ive done just about everything i can, with little progress.
All my linkage and cables are fine, nothing is stuck open.
Took off and Cleaned the BAC, that didnt solve the idle problem at all.
I still have found no vacuum leaks...

I finally got to test the tps with a meter and here is what i got.

Narrow range - 0.94 (which is normal, between the .8 and 1.2)
Full range - 1.27 A little high (supposed to be between .6 and .9 correct??) but from what ive read the full range is only for metering the OMP and wouldn't effect my idle anyways.

Also tested the voltage and got 1v even on the narrow range (again normal) but i got 1v on the full range as well which is supposed to be around .8

Would that little difference cause the problem?? should i replace it?

 

i tried tps adjustment using voltage and never seemed to get it. but i finally used the k ohm settings and it seemed better.

 

When you test it using the voltage the car needs to be in the on position (just like doing the ohm, or the light test) and it has to be plugged to the ECU Still.

 

As for trying to "get the idle down" by adjusing the stop screw on the TB, don't do it. That screw should never need to be touched. As for timing, remember the OP said the symptoms happened suddenly. The idle screw didn't "suddenly" unscrew itself, and the CAS didn't "suddenly" move by itself either. Things that happen suddenly to affect idle are usually things like vacuum hoses that split open and suck air (like the small ones at the rear of the rat's nest, above the exhaust manifold), TPS, coolant temp sensor, boost sensor, thermowax, things like that. I would first focus on vacuum leaks and TPS. I prefer to use the propane method of checking vacuum leaks.

 

Ive already adjusted the TPS, and ive been checking for vacuum leaks using the propane method but ive found nothing. Im probably gonna take it to have someone run a vacuum test on it cause if there is one, i sure as hell am not going to find it.

Any comments on the numbers i got from the TPS?

 

According to your readings, the TPS looks to be ok. To check the BAC for sticking open, use a pair of needlenose pliers to pinch off the large hose that goes from it to the intake duct. If the idle stops surging and slows way down, then suspect the BAC. If no change, start pinching other vac hoses closed and look for rpm changes and report back.

 

I just took off and cleaned the BAC so i dont think thats it, but ill try that then start on the vacuum hoses.

I guess i should also mention that when i unplug the TPS it also stops the surging and sits at a steady 900 rpm (still not the 750 it was at before the sudden problem) so..

 

The one thing about the TPS is that the carbon substrate inside it breaks down over time which can result in inconsistent voltage readings mostly at idle and just above idle (most time spent driving is done in that range) which can make it a bit tricky to determine if it's the root cause. Does your idle ever go to the correct speed or is it consistently too high and fluctuating?

 

see, thats the thing. sometimes it will sit steady, and then if i tap the gas it will start to surge again.

right now with the screw on the TB all the way closed, once it is warm it rarely surges but it will sit either at 1400 rpm, or if i tap the throttle it will sit at 900??

 

Could still be the TPS. I would check narrow-range volts in both instances where the idle is high and when it's normal. When it's too high can you manually move the throttle closed by hand to slow it down? the reason I ask is the dashpot could be preventing it from closing fully.

 

I checked the dashpot, although its kind of stiff its not preventing the throttle from closing. i can push the throttle a little bit (with some force) to get the idle down and smooth, but i dont see anything in the way. im probably just bending it closed if anything.

All the numbers are within normal range when its normal, and when its surging. although slightly higher when its surging (narrow range .94 - .99 full range 1.27 - 1.29)

I just went to check if pinching of the tube to the BAC did anything, which it didnt, but i was wondering what the second tube from the intake went to? that goes under the UIM. because i accidently pinched that off (trying to get to the BAC) and that slowed my idle down to about 700.

 

That hose goes to the oil injector air bleeds (vacuum "spider"). You will likely get some rpm change when you pinch that one closed, so disregard that one.

 

voltage was also the same, regardless if it was surging or not. 1v even for the narrow range, and .99v for the full range.

 

I wonder if your thermowax (if you still have yours) is preventing the roller from moving away from the fast idle cam? As they age, small amounts of wax leak out and it doesn't move full-stroke as when new. It's difficult to check on an NA, but still a possibility. Anything that holds the throttle open at an idle above 1100 rpm and the ECU will retard timing to bring the speed back down, and when it comes back down, the timing advances again, bringing the rpm back up again, and it repeats this, causing the rolling or surging idle. This can be verified with a timing light at the crank pulley.

 

I found a pic that I had when I helped out another forum member. It shows the t-wax and roller. The roller face is green in the pic:

[IMG][/IMG]

 

im gonna have to wait to get my timing light back before i can check i hate lending things out..

Quick question tho, ive been seaching through that damn manual for about an hour and i cant find it. What is the tube on the back of the dynamic chamber connect to? The one right above the VDI?